Oriented magnetic steel plate is mainly used for stationary induction apparatuses such as transformers. As characteristics to be satisfied, (1) a small energy loss, that is, watt loss, when excited by AC, (2) a high magnetic permeability and easy excitation in the excitation range used of equipment, (3) a small magnetostriction due to noise, etc. may be mentioned.
Regarding the watt loss, a transformer is continuously excited and energy loss continues to occur over a long period from installation to disposal, so this becomes an important parameter determining the TOC (total owning cost)—an indicator of the value of a transformer.
To reduce the watt loss of oriented magnetic steel plate, numerous technologies have been developed up to now. That is, there have been (1) raising the density of the {110}<001> orientation called the “Goss orientation”, (2) raising the content of the Si and other solute elements for raising the electrical resistance, (3) reducing the plate thickness of the steel plate, (4) giving a ceramic coating or insulating coating giving surface tension to the steel plate, (5) reducing the size of the crystal grains, (6) introducing strain or grooves in a line form so as to divide the magnetic domains, etc. Regarding (6), Japanese Patent Publication (B2) No. 57-2252 discloses a method of lasering steel plate, while Japanese Patent Publication (B2) No. 58-2569 discloses a method of introducing mechanical strain in the steel plate, various methods of dividing the magnetic domains, and a material exhibiting superior watt loss characteristics.
On the other hand, for the magnetic permeability and magnetostriction, raising the orientation density of the crystal grains to the Goss orientation is effective. The magnetic flux density at the excitation force of 800 A/m, that is, B8, is used as an indicator of that. As one of the typical technologies for improving the magnetic flux density, the method of production disclosed in Japanese Patent Publication (B2) No. 40-15644 may be mentioned. This is a method of production making AlN and MnS function as inhibitors inhibiting crystal grain growth and making the rolling ratio in the final cold rolling process a strong rolling ratio over 80%. Due to this method, the orientation density of the crystal grains in the {110}<001> orientation rises, and oriented magnetic steel plate having a high magnetic flux density of a B8 of 1.870 T or more is obtained. Further, as technology for improving the magnetic flux density, for example, Japanese Patent Publication (A) No. 6-88171 discloses the method of adding, in addition to AlN and MnS, 100 to 5000 g/ton of Bi to the molten steel to obtain a product with a B8 of 1.95 T or more. However, if using the method of using these Al-based inhibitors to raise the magnetic flux density, it is known that the adhesion of the primary coating having a forsterite coating as its main ingredient (hereinafter referred to simply as a “coating” in the present invention in some cases) particularly deteriorates.
In this regard, at the time of the final annealing of the oriented magnetic steel plate, usually an annealing separator having MgO as its main ingredient is used. Adding additives to these so as to improve the magnetic characteristics, coating adhesion, and other various characteristics of oriented magnetic steel plate has been proposed.
Japanese Patent Publication (A) No. 60-141830 discloses a method of production of oriented silicon steel plate adding to an annealing separator having MgO as its main ingredient one or more of additives selected from La, La compounds, Ce, and Ce compounds in a total weight as La and Ce compounds of 0.1 to 3.0% with respect to the MgO and adding S or S compounds in an amount as S with respect to the MgO of 0.01 to 1.0%.
This discovers that by ensuring the copresence of La and Ce with a strong affinity with S, the inhibitory action on the grain growth of primary recrystallization and the action of strictly controlling the orientation of the secondary recrystallized grains grown from the surface layer result in striking improvement of the magnetic characteristics. However, the steel slab ingredients described in that publication do not contain Al effective for realization of a high magnetic flux density. The effect of Al, which has a great effect on the adhesion of the primary coating, is not alluded to.
Further, Japanese Patent Publication (B2) No. 61-15152 discloses an annealing separator for grain-oriented silicon steel strip using magnesium oxide as a base material, said annealing separator characterized by including a rare earth oxide alone or together with a metal silicate. Further, this discloses that a product free from small discontinuities (recessed parts of small holes) below the skin of the strip is obtained and a low magnetostriction rate and good surface resistivity and adhesion are obtained. However, that publication does not touch upon the effects of deterioration of the adhesion of the primary coating seen in particular when using an Al-based inhibitor at all.